The crustal and megathrust events are situated south of Jakarta at distances 85 km and 200 km and with magnitudesMw 6.5 andMw 9.0, respectively. The megathrust is dipping north while the crustal fault is dipping south. The third scenario simulates propagation of seismic waves originating from a medium-depth intraslab earthquake located at the depth of the subducting slab at 180–204 km (fault width = 34 km) directly beneath the city. This fault is dipping south and the earthquake has magnitude Mw 7.0.
Snapshots from the crustal fault scenario are presented in Figure 5.12. After 10 s, the P-wave is showing up in the lower left corner of the topmost panel and at 12 s, both P-wave (dark) and S-wave (vermilion) are observed approaching the basin. At 50 s, seismic waves, both body and surface waves, have entered and are trapped inside the basin. Surface waves are modulated inside the basin and at 90 s, while body waves are attenuated and have faded away outside the basin, surface waves are still reverberating in the basin.
Seismograms (horizontal component) resulting from these three scenarios are pre- sented in Figure 5.13a–c. Two types of seismograms are plotted: (1) those calculated using the elastic parameters indicated in Table 5.1, colored blue in Figure 5.13 and referred to here as ’basin seismograms’, and those calculated using an identical computational mesh but with the basin elastic parameters replaced by those of the basement (i.e., the Basin parameters in Table 5.1 are replaced by those of Layer 2), colored orange in Figure 5.13 and referred to here as ’bedrock seismograms’. The three record sections in Figure 5.13a–c clearly show that seismic waves propagating through the soft sediment inside the basin are amplified to different degrees. Outside the basin the orange colored curves (bedrock seis- mograms) match the blue curves (basin seismograms) perfectly, meaning that outside the basin, no amplification is observed. On the other hand, inside the basin, basin seismograms have much higher amplitudes and prolonged durations in comparison to bedrock seismo- grams. It is interesting to note that the basin-bedrock seismogram ratio is not uniform, and basin depth is not the only factor contributing to the amplification. Basin geometry and direction of incoming waves also appear to influence the degree of amplification.
For the crustal earthquake scenario, Figure 5.13a, seismic waves propagating toward the north edge of the basin are reflected back into the basin and recorded at 200 s at the southernmost station (S2117) and at progressively earlier times at more northerly stations. However, at S2157 to S2176, reflected waves are not clearly seen because they interfere with seismic waves propagate northward, producing high amplitude seismograms at 50–60 s (Figure 5.18). The megathrust earthquake also exhibits reflected waves that are clearly observed at S2130 to S2169, again with reflected waves recorded earlier in the north than in the south (Figure 5.13b).
In contrast to the other two scenarios, the intraslab earthquake scenario shows reflected waves from both south and north edges. Near the south edge, high amplitude seismic waves are observed at stations S2130-2135 at times 50–100 s. These high am- plitudes are generated from interaction between incoming and reflected waves as well as trapping at the basin’s edge. As time goes by, waves reflected by the northern edge of the basin are recorded after 100 s in the southern stations and recorded earlier in the central and northern stations.
Seismograms in Figure 5.14a,b record incoming P- and S-waves at 20 s and 36 s, respectively, for the crustal fault scenario. For the basin seismograms in Figure 5.14c,d, the direct S-wave is followed by a series of reverberations comprised of S-wave
Figure 5.12: Snapshots of wave propagation, showing waves approaching (10 and 12 s, top two panels) and reverberating inside (50, 70 qnd 90 s, bottom three panels) the basin. The modeled Mw 6.5 earthquake is taken to have ruptured a southward dipping, shallow crustal thrust fault 85 km south of the city center. The Jakarta Basin is the light gray colored area, overlying the dark grey medium, clearly shown in the top two pictures capturing snapshots at 10 and 12 s, respectively.
and Rayleigh wave energy, that builds up over the following 15 s, with the highest vertical component amplitude achieved 10 s after the direct S-wave arrival. It is observed that S-wave/Rayleigh wave coda that builds up at about 37 s is still observed after more than 150 s. The bedrock seismograms (Figure 5.14e,f) are dominated by the direct S-wave and have a very weak coda after only a few seconds.The long duration (>120 s) and very high amplitude of basin seismic waves after 40 s is likely due to the constructive interference between seismic body waves and surface waves.
The intraslab scenario produces similar results, at the same station, with surface waves observed after 55 s and still trapped inside the basin after 240 s. In the case of the megathrust event, the Rayleigh wave arrives about 25 s after the P-waves recorded in the seismograms. The interference of reverberating surface waves leads to very high amplitudes, compared to the crustal and intraslab scenario. Entrapment of seismic waves inside the basin prolongs the duration of seismic waves, with high amplitude seismic waves still observed 10 minutes after the earthquake. Interference between seismic body waves and secondary surface waves was recognized as a main cause of building collapse in Kobe during the 1995 Great Hanshin Earthquake (Zhao et al.[2010]).
The three scenarios indicate that the larger the magnitude, the longer the seismic waves were observed inside the basin. The “red” (i.e., long-period dominant) spectra of frequency content generated by the larger rupture area may be responsible for the very long duration of long period ground motions generated by the megathrust scenario (Fig. 5.12). Together with the maximum amplitude and duration of seismic waves, frequency content is also a very important factor that is responsible for building damage. According toShoji et al.[2004], duration is more event-dependent than site-dependent while the site-dependency for a given total power is greater than the event-dependency.
§5.6 Results 101
Figure 5.13: Bedrock seismograms (orange traces) are plotted over basin seismograms (blue traces) for (a) crustal, (b) megathrust and (c) intraslab events, respectively. Labelled points indicate location of stations corresponding to the seismograms plotted directly above the points. In the area between dotted lines (21–41 km from the basin’s rim) the basin structure is inferred from Cipta et al. [2018], while the extension of the basin is estimated from geological data. The basinal area in this figure is the same as the basinal area in Figure 5.12.
Figure 5.14: Seismograms at stations S2169, both for vertical (a) and horizontal (b) components, showing P-, S- and surface waves generated from the crustal fault scenario. Figure (c,d): the same seismograms at time 10–50 second, showing the arrival of direct S followed by Rayleigh surface waves at about 37 s. Similar to (c,d), (e,f) are seismograms recorded at bedrock sites.
§5.7 Discussion 103
Figure 5.15: Design response spectra used for the 2012 Indonesian Building Code for soil sites (classes D and E are brown and black curves, respectively) in Jakarta, compared with average, 1- and 2-σ (solid red, blue dashed and green dashed, respectively) results for simulated response spectra (SRS) calculated in the Jakarta Basin for the (a) megathrust (GMPE:AEA2015); (b) crustal (GMPE: CY2014) and (c) intraslab (GMPE:AEA2015S)earthquake scenarios. Each curve represents the results of calculations at all sites for whichVS30 andZ1.0, have been estimated, as
well as 1000 ground motion realizations that have sampled the aleatory variability in the respective GMPEs.